Bar code field and bar code reader

ABSTRACT

An article of manufacture contains a substrate which contains a bar code field containing bar elements and background elements. The bar code elements have embossed, optically-reactive, microscopic periodic, relief structures which are suitable for optical-machine readout. A readout device generates an incident light beam and contains photosensors to detect the diffracted light beams.

FIELD OF THE INVENTION

The present invention relates to an article of manufacture including asubstrate for supporting a bar code field arrangement. Moreparticularly, the present invention relates to a bar code fieldcontaining encoded embossed relief structures for optical machinedecoding. The information is optical machine readable by means ofreflected or transmitted light, diffracted through angles and alongdirections corresponding to the data encoded in the embossed reliefstructures. The embossed relief structures have periodic andorientational variations which make them suitable for use in securityand authenticity applications.

BACKGROUND OF THE INVENTION

A bar code field is generally known in the retail trade, for example.Such bar code fields are suitable to mark merchandise of all kinds andcontain in coded form numeric information in the form of bar elements ofdifferent widths which are typically laid out across the long side ofthe bar code field. Depending on the application, different types of barcodes are used, e.g., according to MIL-STD-1189 or according to the"European Article Numbering Code".

The support material is most often paper, and the bar elements in thebar code field are therefore applied on a support by means of a simpleprinting process in a color that contrasts with the background.

Optical readers are known from U.S. Pat. No. 4,743,773 for example. Theoptical reader scans the bar code field in the longitudinal direction bymeans of an incident light beam, records the intensity values of thereflected radiation, converts these values into electric signals andtransmits these to an evaluating device. The bar code is self-timing, sothat the reader and the evaluating device are able to recognize therelative widths of bar elements following each other even when thereading speed is changed constantly. The evaluating device convertsthese signals into an appropriate code (e.g. binary number) and into animpulse signal by means of a predetermined algorithm for the purpose offurther processing.

The French patent 2,490,848 and the U.S. Pat. No. 4,743,744 describesuch systems comprising an optical reader and of an evaluating devicewhich are able to read a bar code from the bar code field.

For certain applications the ease with which the bar code field can beproduced is of great advantage. On the other hand it would be desirableif the bar codes could be used for the identification of documents suchas bank notes for example, because of the simple and reliable readoutafforded by bar codes. However, the ease of production rules out theapplication of conventional bar codes in such cases.

Furthermore, machine-readable diffraction-optical markings which aredifficult to copy are known. They are for example embossed in the formof microscopic relief structures into a thin thermoplastic layer appliedon paper, are provided with an optically active layer and are protectedwith a transparent coating material.

The relief structures can have cross-sections of known periodicfunctions with special frequencies of over 10 lines per millimetereffective for the diffraction of visible light. Limits due tomanufacturing restrict the practically useful range to approximately2,500 lines/mm. But cross-sectional forms with aperiodic functions whichcontain local spacial frequencies in that range, such as for examplematte structures, are also usable. The difference in height of theserelief structures is typically selected to lie between 50 nm and 10,000nm.

Similar relief structures diffract incident light and containauthenticity information, for example in the form of images, the colorsand luminosity of which depend on the viewing angle or the movementeffects of which depend on the change of the viewing angle.

Such documents and processes for their manufacture are described inSwiss patent 594 936 and in Swiss patent application 00805/88-4.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the instant invention to provide an article ofmanufacture including a substrate or support for supporting a bar codefield which is difficult to copy. Typically, the article of manufactureis a valuable document such as a bank note. Because the bar code fieldof the present invention is difficult to copy, it is suitable for themarking of bank notes and other valuable documents.

Illustratively, the object of the invention is realized by an opticalmachine readable bar code which comprises embossed, microscopic reliefstructures in each bar code element. A substrate, modified by themicroscopic relief structure, is rendered optically reactive and willdiffract an incident light beam at an angle and along a directionuniquely determined by the characteristics of the relief structure. Thediffracted light beam, which can be transmitted or reflected, dependingon the properties of the substrate, is read and decoded byoptical-machine means.

Embodiments of the invention are explained below in greater detailthrough the drawings.

DESCRIPTION OF THE DRAWING

FIG. 1 shows an embodiment of a bar code field with diffractionelements, in accordance with an illustrative embodiment of the presentinvention.

FIG. 2 shows an arrangement of light beams during readout of the barcode field of FIG. 1.

FIGS. 3 and 4 show an arrangement of light beams during illumination andreadout of diffraction elements having different azimuth angles.

FIG. 5 shows an embodiment of a reader for reading bar code fieldscomprising diffraction elements.

FIG. 6 shows an embodiment of the present invention in which twosuperimposed bar code fields are utilized.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 a bar code field 1, a support or substrate 2 for the bar codefield and an object or article of manufacture 3 (e.g. a valuabledocument) which is marked with the bar code field 1 are delimited on oneside by a cut 4. The support 2 can be made in form of a self-stickinglabel, for example, and be bonded so firmly to the object 3 that saidbar code field is destroyed if an attempt is made to loosen it. Inanother embodiment, the support 2 comprises a thin thermoplastic colorlayer applied on paper or of a film formed in a hot embossing process,as is known from Swiss patent 594,936.

The bar code field 1 comprises a number of encoded markings which aremade up in turn by a predetermined number of rectangular bar elements 5and background elements 6 with different widths B in the direction ofreadout.

The bar code field 1 is therefore divided into at least two partialsurface groups. The bar elements 5 of the first partial surface groupare hatched for clarity in the figures. Two adjacent bar elements 5 areseparated by at least one background element 6. The background elements6, together with the second partial surface group, constitute thebackground against which the bar elements 5 of the bar code field 1stand out.

If at least one additional partial surface group is used, with partialsurfaces alternating with the elements 5 and 6 in the sequence in barcode field 1, the possibilities of encoding are increased.

Furthermore a lateral border surface 8, which can delimit the bar codefield on at least one longitudinal side 7 of the bar code field 1, isrepresented in FIG. 1 by a double line.

In each partial surface group as well as in the lateral border surfaces8 if these are used, microscopic, predetermined relief structures areembossed or imprinted, for example by the method described in Swisspatent 664,030.

These relief structures are generally periodic and are given a spacialfrequency f and azimuth angle φ as their most important parameters whichdetermine the form and dimension of the relief profile.

FIG. 2 shows details of the relief structure and readout arrangement.

The azimuth angle φ defines the orientation of a relief structure (e.g.12 of FIG. 2) in relation to the bar code field 1 and constitutes theangle between the longitudinal side 7 of the bar code field 1 and adirection 9 of a grating vector of a relief structure (e.g. 12).

The printed bar code known in the retail trade utilizes the differencesin absorption of the readout light as the sole parameter. In contrast tothis binary system, the bar code with embossed relief structures has upto three parameters that are independent of each other, i.e., even anoctal base is possible. The greater number of parameters used todifferentiate the relief structures makes it possible to includeadditional security information in the same bar code field, for example.

Each of the three parameters used for the identification of the reliefstructure alone as well as their combinations can be used to build up anencoding in the bar code field 1, and a person schooled in the art wouldeasily be able to derive other types in addition to the embodimentspresented here.

It is possible, in particular, to use superimposed relief structureswith at least two different spacial frequencies f_(A) and f_(B) and ofdifferent form and dimensions, for example with f_(A) being in the rangebetween 10 and 200 lines per mm and f_(B) in the range between 50 and2500 lines per mm.

The borders of the different relief structures of the bar code field 1which are shown in FIGS. 2 and 3 are limited only by the drawingpossibilities, since the surface portions 5, 6 and 8 differentiatethemselves only through the parameters of the embossed reliefstructures.

To read out the information contained in a relief structure, an opticalreader is used which can be guided freely manually over the bar codefield 1 or which is provided with a conveying device (not shown) for theobject 3. The reader produces a readout light beam 10 (see FIG. 2) withlight in the visible or infrared range of the spectrum.

The relief structures are readable in transmission or in reflection,with an optically active intermediary layer according to Swiss patent594,936, the material of the support 2 and the object 3 determining thebest type of readout.

In reading the information in reflection, at least the coating materialof the bar code field 1 is transparent to the readout light beam.

In reading the information in transmission, the support 2 and thecoating material of the bar code field 1 are transparent to the readoutlight beam 10, with the surface of the microscopic relief structurebeing an interface surface between two materials with different indicesof refraction. The object 3 is provided with a recess in the area of thebar code field 1 or is also transparent to the readout light beam 10.

The relief structures of the partial surface groups and the lateralborder surfaces 8 modify the environmental light which they receive.According to the predetermined transparency of the coating material ofthe bar code, the bar code is visible to the eye in the environmentallight or remains completely invisible to the naked eye. Formachine-reading, an optical transparency of the coating in thewavelength range of the readout light suffices. For example, a visiblemarking of the bar code field 1 printed on the coating material is usedadvantageously to locate the bar code field 1, since in manual guidanceof the optical reader the bar code field 1 is easy to locate and theinformation is easy to read out parallel to the longitudinal side 7.

The reader is provided with an optical installation 11 which forms thereadout light beam 10. An illuminated relief structure 12 of the barcode field 1 therefore has a predetermined form, e.g., that of a circle,of a rectangle or of an ellipse, of which the dimension in the directionof reading, i.e., parallel to the longitudinal side 7 is smaller thanthe least width B of the elements 5 or 6. When a manually guided readeris sued, the form is preferably a circle in order to decrease therequirement that reading be done parallel to the longitudinal direction7. If on the other hand the object 3 is moved by means of the conveyingdevice parallel to the longitudinal direction 7, the rectangular form ispreferable.

For machine-recognition it is preferable for the readout light beam 10to be of one color to differentiate between different diffractionstructures. It can be produced by means of an inexpensive semiconductorlaser for example, which is used as a light source 13.

The reader guides the readout light beam 10 from beginning to end overthe bar code field 1, parallel to the longitudinal side 7, whereby oneof the elements 5 or 6 is being illuminated alternately. Every reliefstructure of both partial surface groups diffracts the readout lightbeam 10 which preferably falls perpendicularly on the bar code field 1in a predetermined manner which is determined by the illuminated reliefstructure 12.

The illuminated relief structure 12 diffracts the readout light by adiffraction angle θ dependent on the wave length of the readout lightand on the spatial frequency f. As shown in FIG. 2, light 14 isdiffracted under diffraction angle θ into two symmetric directions (θ,φ) and (θ, φ+180°) in relation to the perpendicularly falling readoutlight beam 10, whereby the diffracted light 14 and the readout lightbeam 10 define a plane 15 containing also the grid vector direction 9 ofthe illuminated relief structure 12 in the case of the perpendicularlyincident readout light beam 10. The longitudinal side 7 with the plane15 therefore also form the azimuth angle φ.

The relief structures of the bar code field 1 (FIG. 3) can have thesame, predetermined azimuth angle φ₁ for each bar element 5 for example.It differentiates itself from the predetermined azimuth angle φ₂ whichis common to all background elements 6, by the difference δ. When theperpendicular incident readout light beam 10 is shifted parallel to thelongitudinal side 7 from a bar element 5 to a background element 6, theintensity of the light 14 diffracted in the directions (θ₁, φ₁) and((θ₁, φ₁ +180°) decreases in plane 15 as a function of the illuminatedsurface portion of the bar element 5 while the intensity of alight 14diffracted in a direction (θ₂, φ₂) and (θ₂, φ₂ +180°) increases in a newplane 16 as a function of the illuminated surface portion of thebackground element 6. The orientation of the plane 16 is defined by adirection 17 of the structure vector of the background element 6 and bythe readout light beam 10. The two planes 15 and 16 therefore also formthe angle δ. As soon as the readout light beam 10 only illuminates thebackground element 6 the diffracted light 14 can be found only in plane16.

If the bar code field 1 contains additional partial surface groups withother orientations of the relief profile, these other relief profilesdefine additional planes 16 which differ from the plane 15 by variousangles δ.

If the relief profile has a symmetric configuration the intensities ofthe diffracted light 14 are distributed in the planes 15 or 16symmetrically in relation to the perpendicular incident readout lightbeam 10 falling on the relief structure 12.

If on the other hand the relief profile has an asymmetric configuration,the intensity of the diffracted light 14 is greater in one predetermineddirection than in the other as a function of the asymmetry.

As shown in FIG. 4, the reader comprises at least one photosensor 18.The light 14 diffracted at the illuminated relief structure and fallinginto the photosensor 18 and the readout light beam 10 form thediffraction angle θ and define a readout plane 19. It is advantageous touse two photosensors 18 as a pair, arranged in readout plane 19symmetrically in relation to the readout light beam 10 at the angle θ.This makes it possible to ascertain the symmetry or asymmetry of therelief structure 12.

If the relief profile has an asymmetric configuration, the twophotosensors 18 of a pair measure advantageously the intensitydifference between the light 14 diffracted in direction (θ, φ) and indirection (θ, φ+180°), e.g., in order to decrease the effects ofextraneous light.

The bar code field 1 is preferably provided with a relief profile withan asymmetric configuration and is part of a graphic representationformed with the parameters spacial frequency f and azimuth angle φ ofthe relief structure. The bar code with the elements 5 and 6superimposed on the image defines the asymmetry of the relief profilefor these two partial surface groups.

As shown in FIG. 4, it is also possible for the reader to be providedwith at least one additional readout plane 20 to record the light 14diffracted in that readout plane 20 by another relief structure. Ifthese relief structures have an azimuth angle difference of δ, thereadout planes 19, 20 also form preferably the difference angle δ.

In reading the bar code, the readout light beam 10 desirably fallsperpendicularly on the plane of support 2 while the readout planes 19,20 defined by the photosensors 18 coincide with the planes 15, 16 (FIG.3) defined through the diffraction process. Additional deviations fromthese conditions are to be expected with a thin support 2 on a non-flatobject 3, e.g., on a crumpled bank note. Here the readout light beam 10varies in different, non-defined ways from the perpendicular to thesurface element from one surface element of support 2 to the other.

The photo-detectors 18 (FIG. 4) of the reader have preferably a wideacceptance angle and record all of the light 14 diffracted in a solidangle Ω. This can be achieved by means of a photosensor 18 with a largelight-sensitive surface or by means of an optical means 21 upstream ofthe photosensor 18 which concentrates the diffracted light 14 on apoint-shaped photosensor 18.

A lens or mirror system or an optical diffraction focusing element canbe used as the optical means 21 and for the optical device 11. Theoptical diffraction element has a relief structure that is similar to adiffraction grating but due to the form and dimensions of the reliefstructure it is able to focus light that is incident at a predeterminedangle in a predetermined focal point. Such optical focusing elements areknown in transmission and reflection, such as for example holographicoptical elements (HOEs).

In another embodiment of the bar code field 1 the elements 5 and 6 aredifferent in that only one of the two partial surface groups has aperiodic relief structure, e.g. the bar elements 5, while the backgroundelements 6 are engraved with a matte structure.

Simple straight-line diffraction gratings with symmetric or asymmetricrelief profiles are advantageous, but other types of relief structures,the form of which is only limited by the embossing or imprintingtechnology, can also be used.

In particular uni-directional matte structures can be used. These aregrating structures, the grid constant of which varies statistically fromlocation to location, whereby the elements 5 and 6 are differentiatedthrough the azimuth of the relief structure.

For example, the parameters of the gratings, whose elements 5 and 6belong to N different partial surface groups, differentiate themselvesonly in their N spacial frequencies F₁, F₂ etc. and all have the sameazimuth angle φ. The light 14 diffracted by the elements 5, 6 etc. thenappears under the N diffraction angles θ₁, θ₂ in accordance with thenumber N of the partial surface groups. The photosensors 18 of thereader for such a bar code field 1 have the largest possible acceptanceangle, whereby the range within the azimuth angle amounts to a full 360°and the range of the diffraction angle θ is divided into N angle ranges.

According to FIG. 5 the reader has advantageously a symmetric structureand can be built into a hollow cylinder 23 for example, on the axis 22of which the illumination source 13 with the optical device 11 isarranged so that the readout light beam 10 is cast in a direction alongaxis 22 on the illuminated relief structure 12 of the bar code field 1located outside the hollow cylinder 23. A distancing device (not shown)guides the reader at a predetermined distance across the bar code fieldin such manner that the optical axis 22 is nearly perpendicular to thecentral plane of the bar code field 1. The diffracted light 14 falls onthe optical means 21 installed all around on the inner wall of thehollow cylinder 23. They reflect the light 14 diffracted at thediffraction angles θ₁, θ₂, etc. in a focusing manner on one of the Nphotosensors 18 which are located behind the illumination source 13 inone of the focal points corresponding to the N diffraction angles θ₁,θ₂, etc. on axis 22. This reader does not have a direction marked byazimuth and is suitable for manual guidance.

It is possible for the parameters of the gratings of the elements 5, 6etc. of the partial surface groups to be differentiated only withrespect to the value of their azimuth angle φ. For example, such a barcode field 1 has two partial surface groups. The difference δ betweenthe azimuth angle φ of the two grids is δ=90°, whereby the azimuth angleφ for the grids of the bar elements 5 has a value of φ₁ =45° and has avalue of φ₂ +-45° for the gratings of the background elements 6. Thereader (FIG. 4) suitable for this bar code field 1 has a range from 20°to 80° for the acceptance angle for each photosensor 18 in the readoutplanes 19,20 for the angle θ, and for the azimuth angle θ has a range of±40° and is also suitable for manual guidance.

The relief structures of the elements 5, 6 have preferably no constantspacial frequency f₁, f₂ but are modulated with a predetermined spacialfrequency amplitude A, whereby A is smaller than one half the differencebetween adjoining spacial frequencies f₁, f₂. In the illuminated reliefstructure 12 all the spacial frequencies within a frequency rang f±A aretherefore active. This causes a predetermined fanning of the diffractedlight 14 in the plane 15, 16 and increases the reading reliability whena support 2 is not entirely even, for example, if the incident readoutlight beam 10 is not precisely perpendicular.

The bar code field (FIG. 2) can be provided with the lateral bordersurface 8 at least at one longitudinal side 7. A grating with the sameparameters as the grating of the background elements 6 can for examplebe embossed into the lateral border surface 8. However, the grating ofthe lateral border surface 8 can also be different from the two reliefstructures of the element 5, 6.

The two gratings of the lateral border surfaces 8 can have a spacialfrequency f_(s) and be different from each other in the two azimuthangles φ_(s) by a difference α. The gratings of the elements 5, 6 arepredetermined by the spacial frequency f and the azimuth angles φ₁, φ₂.Advantageously the azimuth angle φ_(s) has the same value as the azimuthangles φ₁, φ₂ of the elements 5, 6 so that the reader may not be limitedto the azimuthal direction. A part 10' (FIG. 2) of the rectangularreadout light beam 10 illuminates the lateral border surfaces 8. Lightis diffracted in the form of rays 24 at an angle β which is differentfrom angle θ in a plane assigned to azimuth φ_(G) and parallel to plane15.

A reader for this design of the bar code field 1 (FIG. 4) isadvantageously provided in each readout plane 19, 20 with pairs ofphotosensors 18, 25 placed symmetrically in relation to the readoutlight beam 10, whereby the photosensors 18 and the light 14 diffractedat an angle θ register and whereby the photosensors 25 receive the rays24 diffracted at an angle β.

As an example, the optical means 21 of the photosensor 18 limits theusable solid angle Ω of the diffracted light 14 to a range of thediffraction angle θ between 20° and 45°, while the optical means 21 ofthe photosensors 25 accept the rays 24 within the range of angle βbetween 46° and 80°.

It is also possible for the bar code field 1 (FIGS. 1 and 2) to beprovided with at least one lateral border surface 8 the longitudinalborder 26 of which has a predetermined form on a side away from theelements 5, 6, whereby the longitudinal border 26 is configuredadvantageously as a sine, rectangular, sawtooth or other periodic wavefunction with a predetermined wavelength L. The intensity of the rays 24change therefore in function of an illuminated, variable surface portion27 of the lateral border surface 8.

The wavelength L is advantageously a constant and monotone function anddepends in a predetermined manner on the location on the longitudinalborder 26. The instantaneous position of the readout light beam 10 inthe bar code field 1 can be determined by a continual determination ofthe wavelength L from the intensity of the rays 24.

If two longitudinal border surfaces 8 are present, the wavelengths L ofthe two longitudinal borders 26 can be different.

The photosensors 25 transform the intensity of the rays 24 (FIG. 4) intoelectric signals, so that the wavelength functions of the longitudinalborder 26 may be determined and the location of extreme values of thewave functions can be compared with the position of the encoded markingsof the bar code field 1. The intactness of the bar code field 1 can thusbe checked and each of the two lateral border surfaces 8 fulfills asecurity function.

In the bar code field 1 with a security function shown in FIG. 6, asecond bar code 28 is superimposed on a first bar code. The first barcode is formed from the elements 5, 6. The second bar code 28 is made ofbar surfaces 29 and neutral surfaces 30 and divides the bar code field 1longitudinally into fields that follow each other and to which apredetermined number is assigned by the bar code 28. These numbers ofthe bar code 28 represent, for example, numbers for the numbering ofadjoining fields in the bar code field 1. The bar code of the elements5, 6 and the bar code 28 can also be encoded according to two differentstandards.

In the drawing, the different relief structures are represented bycomposite hatching, whereby the bar code 28 is shifted laterally inrelation to the elements 5, 6 of the bar code field 1 for the sake ofclarity of the drawing.

This double encoding requires complicated relief structures in the barcode field 1. They are determined by at least two parameters, with afirst parameter, e.g. the spacial frequency f, being assigned to theelements 5, 6 and a second parameter, e.g. the azimuth angle φ beingassigned to the surfaces 29, 30.

As an example the relief structure of the bar code elements 5 has thespacial frequency f₁ and the background element 6 the spacial frequencyf₂, whereby the orientation of the relief structure is predetermined bythe azimuth angle φ₁ or φ₂, according to the attribution to the barsurface 29 or to the neutral surface 30.

In FIG. 4 the reader receives the diffracted light 14 with thephotosensors 18, 25 in each of the two readout surfaces 19, 20, e.g. intwo ranges of the azimuth angle φ and in two ranges of the diffractionangle θ. In the above example, the two photosensors 18, 25 of thereadout surface 19 or 20 obtain the information of the ar code 28(depending on the azimuth angle φ₁ or φ₂). The information of elements 5or 6 are obtained from the signals of the photosensors 18 or 25(depending on the spatial frequency f₁ or f₂) of the two readoutsurfaces 19, 20.

Advantageously, asymmetric relief profiles are suitable in thisembodiment for the bar code field 1. The relief profiles of the elements5 and 6 are different, for example, in the spacial frequency f and arearranged in the bar surfaces 29 according to their asymmetry in a mirrorimage to those of the neutral surfaces 30. The boundaries of thesurfaces 29, 30 and those of the elements 5, 6 should not coincide inthe direction of longitudinal side 7. The elements 5, 6 have thereforesurface portions of both asymmetries in a predetermined manner.

A reader for this embodiment of the double-encoded bar code field 1 isbarely limited in the azimuthal direction with a usable range of ±40°for the azimuth angle φ and has two pairs of photosensors 18, 25 in areadout plane 19 or 20. When reading out the information of the bar codefield 1, the sums of the output signals of each pair of photosensors 18,25 are evaluated. The information of the bar code 28 is contained in thedifference of the output signals of each pair of photosensors 18, 25.

When the signals of the reader are evaluated, evaluation electronics(not shown) can, for example, check the sequence of the surface of barcode field 1, numbered by the bar code 28, and compare the decodedsequences with a predetermined one.

If the readout information does not have an asymmetric disposition aspredetermined by the standard after decoding in the evaluatingelectronics (not shown), said evaluating electronics then checks whetherthe bar code field 1 has been read backwards and resets backward readinformation automatically in the correct direction.

The bar code field 1 with a security function is advantageously usablefor valuable documents, since it has the function of an authenticitycharacteristic and contains information for the classification of thevaluable documents, e.g. concerning value, origin and serial number.

Finally, the above-identified embodiments of the invention are intendedto be illustrative only. Numerous alternative embodiments may be devisedby those skilled in the art without departing from the spirit and scopeof the following claims.

I claim:
 1. An article of manufacture including a substrate forsupporting a first bar code field, said first bar code field beingself-timing and including optically encoded machine readableinformation, said first bar code field comprising bar elements ofdifferent widths separated by background elements, wherein at least saidbar elements each comprise an optically-diffractive microscopic reliefstructures with a spatial frequency of over 10 lines per mm and apredetermined orientation defined by an azimuth angle.
 2. The article ofmanufacture of claim 1 wherein said background elements each comprise amicroscopic relief structure with a spatial frequency and an orientationdefined by an azimuth angle, wherein all the relief structures of thebar elements have an identical first predetermined azimuth angle (φ₁),wherein all the relief structures of the background elements have anidentical second azimuth angle (φ₂), and wherein, the first azimuthangle (φ₁) and the second azimuth angle (φ₂) differ by a predeterminedangle amount (δ).
 3. The article of manufacture as recited in claim 2wherein the relief structures of said bar elements differ from therelief structures of said background elements by an asymmetry in theprofiles of said relief structures.
 4. The article of manufacture asrecited in claim 2 or 3, wherein the spatial frequencies are the samefor all of said microscopic periodic relief structures.
 5. The articleof manufacture as recited in claim 1 or 2 wherein at least one of saidrelief structures is modulated by a spatial frequency of predeterminedamplitude (A).
 6. The article of manufacture of claim 1 wherein saidsubstrate supports a second bar code comprising bar elements andbackground elements and wherein at least one encoded character of saidfirst bar code field has an associated number in the second bar codefield.
 7. The article of manufacture of claim 6 wherein the bar andbackground elements of the first bar code field differentiate themselvesby a first parameter determined by said elements of said first bar codefield by a second parameter determined by the elements of the second barcode field, wherein said first parameter is the spatial frequency of theelements of the first bar code field and the second parameter is anasymmetry of the element of the first bar code field.
 8. The article ofmanufacture as recited in claim 1, wherein said substrate supports alateral surface portion alongside said bar code field, said lateralsurface portion having a longitudinally extending,optically-diffractive, microscopic relief structure.
 9. The article ofmanufacture as recited in claim 8, wherein said relief structure of saidlateral surface portion has a spatial variation in the longitudinaldirection with a predetermined wavelength.
 10. The article ofmanufacture as recited in claim 1, wherein said substrate has atransparent coating and the reading of said bar code field utilizesreflection of incident light.
 11. The article of manufacture as recitedin claim 10, wherein said bar code field is visible through saidtransparent coating.
 12. The article of manufacture as recited in claim1 wherein said substrate comprises a self-adhesive label.
 13. Thearticle of manufacture is a valuable document, and wherein said bar codefield contains information relating to said document and is located at apredetermined region of said valuable document.
 14. An apparatus forreadout of a self-timing bar code field having a predetermined sequenceof bar code elements of different widths separated by backgroundelements for encoding information, each of said bar code elements havingan optically-diffractive, microscopic periodic relief structurecomprising a plurality of lines with a predetermined spatial frequencyand a predetermined angular orientation for diffracting incident light,said readout apparatus, comprising:a light source for generating areadout light beam and for directing said light beam onto said bar codefield, photosensor means for sensing a first diffracted light beamdiffracted by said optically-diffractive, microscopic periodic reliefstructures of said bar code field, said photosensor means being locatedin a first plane defined by said readout light beam and said firstdiffracted light beam at a point where said photosensor means willintercept said first diffracted light beam, said photosensor meansserving to convert said first diffracted light beam into an outputsignal.
 15. The apparatus as recited in claim 14, wherein said apparatusfurther includes optical means located between said photosensor meansand a relief structure illuminated by said readout light beam forfocusing said diffracted light onto said photosensor means.
 16. Theapparatus as recited in claim 14, wherein said photosensor meanscomprises first and second photosensor devices located in said firstplane so that light diffracted at either of a pair of predeterminedazimuth angles (φ or φ+180°) is converted into an output signal.
 17. Theapparatus as recited in claim 14 wherein a second readout plane,non-co-planar with said first readout plane, is defined by a seconddiffracted light beam and said readout light beam and wherein secondphotosensor means are located in said second readout plane for detectingsaid second diffracted light beam.
 18. An article of manufacturecomprising a substrate for supporting a self-timing bar code field, saidbar code field comprising a first sequence of bar code elements ofdifferent widths superimposed on a second sequence of bar code elementsfor encoding information, said bar code elements each having anembossed, optically-diffractive, microscopic periodic relief structurecharacterized by first and second independent physical criterion, saidfirst independent physical criterion corresponding to said firstsequence of bar code elements and said second independent physicalcriterion corresponding to said second sequence of bar code elements.19. The article of manufacture as recited in claim 18, wherein saidfirst independent physical criterion is the spacial frequency of saidembossed, optically-diffractive, microscopic periodic relief structuresand said second independent physical criterion is the degree of profileasymmetry of said embossed, optically-diffractive, microscopic periodicrelief structures.
 20. An apparatus for readout of a self-timing barcode field having a predetermined sequence of bar code elements ofdifferent widths separated by background elements for encodinginformation, each of said bar code elements having anoptically-diffractive microscopic relief structure comprising aplurality of lines with a predetermined spatial frequency and apredetermined angular orientation for diffracting incident light, saidapparatus comprising:a housing, having an axis of symmetry; a lightsource located within said housing on said axis so that an incidentlight beam, collinear with said axis is incident on said bar code field;a first photosensor located on said axis so that said light source isbetween said bar code field and said first photosensor; a secondphotosensor located on said axis so that said first photosensor isbetween said light source and said second photosensor, and axiallysymmetric optic means for directing a first diffracted light beam,diffracted at a first angle with said incident light beam, onto saidfirst photosensor, and for directing a second diffracted light beam,diffracted at a second angle with said incident light beam, onto saidsecond photosensor.